1. Field of the Invention
This invention relates to a method and apparatus for producing synthetic resin sheets with decorative designs and, more particularly, to an improved method for producing synthetic resin sheets incorporating geometric (or regular, as these terms are used interchangeably herein to define a repeating pattern) pattern elements with the pattern image extending throughout the thickness of the resin sheet and comprising the corresponding image portions of the exposed surfaces of the sheet.
2. DESCRIPTION OF THE PRIOR ART
Heretofore, various methods of producing resin sheets having decorative designs have been developed and employed. Examples of methods of producing resin sheets having irregular decorative designs are extrusion methods, calender roll methods and pressing methods.
One of the most popular methods for producing resin sheets having regular decorative designs has included depositing fine particles (of a size of about 2 to about 300 .mu.) of a resin composition on a continuously moving conveyer belt, heating the resin particles to cause a partial welding or fusion together of the individual particles and to form a pre-formed mass having the desired sheet configuration, producing printed geometric patterns on the surface of the pre-formed sheet by means of a suitable printing machine, and applying heat and pressure to the printed pre-formed sheet in a press to form a completely welded, uniform, non-porous sheet structure. This process is hereinafter referred to as the sintering method.
Examples of prior art wherein decorative designs are formed using such a printing apparatus include those disclosed in U.S. Pat. Nos. 3,359,352, 3,574,659, etc.
In another prior art method generally employed, various colored masses of resin particles, generally of a size of approximately 100 .mu. to 2 mm, are deposited in intended compartments defined by a set of mold frames constructed and assembled on a base member and then they are fused and pressed into an integral design sheet. This process is hereinafter referred to as the mold frame method.
The sintering method described above has been used only for the production of relatively thin plastic sheets having regular decorative designs, as it involves the use of a printing apparatus and only a paste or very fine particles (of a size of about 2 .mu. or less) can be used as a printing ink, so that when printing relatively thick sintered porous masses the ink cannot extend over the bottom of the porous masses in a sufficient amount.
Further, when plastic sheets having decorative designs are vertically cut with respect to the surface thereof, the border line of any cut which is between one decorative unit design and another decorative unit design does not appear as a clean straight line extending from the surface to the back side of the sheet. As a result, if a decorative design on the surface of the plastic sheet wears away, it is impossible to retain a semblance of a decorative design the same as that carried on the surface.
Furthermore, in the sintering method described above it is difficult to control the amount of printing ink applied, and powder having as small a particle size as possible is used to form a pre-formed mass (the stage immediately prior to plastic sheet formation) so that particles in the sintered pre-formed mass can effectively absorb printing ink. From this it follows that relatively thick plastic sheets having regular decorative designs cannot be obtained as a final sheet products.
In addition, when plastic sheets have regular pattern elements where the pattern image extends through the thickness of the plastic sheet, any one decorative design unit in the plastic sheet which is obtained by the sintering method described above will not have uniform color extending from the surface to the back thereof.
Also, with the mold frame method described above, a number of problems arise, including smearing due to shifting or sliding of the pattern forming particles used therein during the heat-pressing operation, and variations in the wall thickness of the final sheet product.